When Arduino was conceived AVR was very much the best 8-bit controller available. But we're 10 years on now, and still using the AVR, despite lots of better microcontrollers available, notably ARMs. In particular M0 and M0+ cores are a lot cheaper, yet are a lot faster and have many more features than AVR.
For instance the NXP LPC824 has more serial buses, more A/D channels and higher A/D resolution, is almost twice as fast as the AVR, yet costs 30% less. (Granted, it doesn't have EEPROM, but you can have that for 20 cents.)
So I was wondering if there are particular issues which prevent the use of more modern microcontrollers on Arduino.
One difference may be Vcc, which is 5V for AVR and 3.3V for ARM. But given that more peripherals work at 3.3V this seems more to be an AVR issue than an ARM issue.
PS: I'm aware of Arduino Due, but that one costs twice as much as a Uno. I'm talking about controllers which price-wise can compete with AVR.